1. Field of the Invention
The present invention relates generally to polymer foam processing and, more particularly, to systems and methods for introducing blowing agent into polymeric material in a polymeric foam process.
2. Description of the Related Art
Polymeric materials are processed using a variety of techniques. Many techniques employ an extruder which includes a polymer processing screw that rotates within a barrel to plasticate polymeric material. Some processing techniques, such as injection molding and blow molding, may be discontinuous. That is, during operation, the screw does not plasticate polymeric material continuously. Discontinuous processes may have repetitive cycles which include a plastication period, in which the screw rotates and polymeric material is accumulated, followed by an injection (or ejection) period, in which the screw does not rotate and the accumulated polymeric material is injected into a mold (or ejected through a die).
Polymeric foam materials, including microcellular materials, may be processed by introducing a physical blowing agent into the polymeric material within the extruder through a blowing agent port in the barrel. Many conventional blowing agent introduction systems introduce blowing agents continuously into the polymeric material within the barrel. In discontinuous processes, including certain injection molding and blow molding processes, such continuous introduction systems may cause a lack of control over the percentage of blowing agent injected into the polymeric material and may lead to an uneven distribution of the blowing agent in the polymeric material. In particular, the polymeric material in the vicinity of the blowing agent port, when the screw ceases to plasticate polymeric material, may contain higher amounts of blowing agent because of its increased residence time in proximity to the blowing agent injection port. The uneven distribution of blowing agent may result in viscosity variations within the polymeric material in the extruder which can cause output inconsistencies and other problems. Such effects may generally reduce control over the process and may narrow the processing window.
In some polymer processes, including some discontinuous processes, such conventional blowing agent introduction systems may be adequate. However, in other processes, such as discontinuous processes that require relatively precise control over blowing agent introduction, the conventional introduction systems may impair the process for one or more of the reasons described above. In particular, certain processes for producing small molded articles and/or microcellular foam articles may be adversely affected if the blowing agent is not precisely controlled.